1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method, and particularly to an image forming apparatus forming an image using a transparent toner.
2. Description of the Related Art
In an image forming apparatus employing an electrophotographic method, if images having a low printing ratio (small toner consumption) are output at a high ratio, a toner stays in a developing device and thus becomes deteriorated. More specifically, a developing blade and the toner for forming a toner layer are slid and rubbed on a developing sleeve for a long period, and thus an external additive externally added to the toner may fall off, or the external additive may be buried into a surface of the toner. As described above, if the external additive falls off from the surface of the toner or buried thereinto, a charging characteristic or flowability of the toner is deteriorated (hereafter, referred to as “the toner is deteriorated”). The deterioration of the charging characteristic and the flowability of the toner causes toner scattering and image fogging, which are not preferable.
To address such an issue, Japanese Patent Application Laid-Open No. 2003-263027 discusses that, when the toner is deteriorated at a high ratio, the toner is applied (discharged) to a photosensitive member from a developing device to refresh the toner staying therein.
More specifically, each time the developing sleeve is driven for a predetermined time, an amount of toner consumed in the predetermined time is calculated. When a calculation result is lower than a predetermined value, the deterioration of the toner may be advanced, and thus the toner in the developing device is discharged to the photosensitive member and collected by a cleaner without being transferred to a recording sheet. Further, the toner (new toner) in the amount corresponding to the discharged toner (deteriorated toner) is supplied for the developing device so that the toner does not stay in the developing device for a long period.
Similarly, Japanese Patent Application Laid-Open No. 2006-023327 discusses a method for refreshing the toner in the developing device based on a value (e.g., a video count value for each image formation) indicating the amount of toner to be used for each image formation. More specifically, the method discusses that, when the video count value is smaller than the set predetermined threshold value, a difference therebetween is calculated, and, when an integrated value acquired by integrating the calculated differences reaches the predetermined value, the toner is discharged from the developing device for refreshing.
In recent years, an image forming apparatus has been discussed that uses a transparent toner for adjusting glossiness in addition to color toners of yellow, magenta, and so on.
Usages of the transparent toner are known in which the transparent toner is applied all over a region where an image can be formed of a sheet to give an image glossiness like a silver halide photograph, and in which the transparent toner is locally applied for purpose of forgery prevention or eye-catching (e.g., gloss mark and watermark). However, the image forming apparatus capable of forming an image with the color toner and the transparent toner does not always form the image using the transparent toner.
In other words, the image forming apparatus outputs the image having high glossiness such as the silver halide photograph and forms the gloss mark using the transparent toner according to an image forming mode selected by a user. Thus, unless the user always selects a mode for using the transparent toner, a consumption amount of the transparent toner tends to become smaller than that of the color toner.
If the consumption amount of the transparent toner is smaller than that of the color toner, the transparent toner tends to be deteriorated sooner compared with the color toner. Thus, in a state where the transparent toner becomes deteriorated sooner than the color toner, a down time occurs due to frequent discharge of the transparent toner from the developing device, so that productivity may be lowered.